Vapor rectifier apparatus



July 18, 1933.

S. WIDMER VAPOR RECTIFIER APPARATUS Filed Oct. 15, 1927 5 Sheets-Sheet l Fig.2.

Fig. 3.

July 18, 1933. s. WIDMER VAPOR RECTIFIER APPARATUS 3 Sheets-Sheet 2 Filed Oct. 15, 1927 Fig-4.

July 18, 1933. s. WIDMER 1.918577 VAPOR RECTIFIER APPARATUS Filed Oct. 15, 1927 3 Sheets-Sheet 3 Patented July 18, 1933 UNITED STATES STEFAN \VIDMER, OF BADEN, SWITZERLAND,

ASSIGNOR TO AKTIENGESELLSCHAFT BROWN, BOVERI & CIE, OF BADEN, SWITZERLAND, A JOINT-STOCK COMPANY OF SWITZERLAND VAPQR RECTIFIER APPARATUS Application filed Getobcr 15, 1927, Serial No. 226,491, and in Germany October 18, 1926.

This invention relates to power rectifiers of the metallic vapor type employing a plurality of anodes.

The general object of the invention is the provision of means whereby the capability of the rectifier to carry heavy currents at high voltages is enhanced and its reliability and certainty of operation greatly increased.

A specific object of the invention is the prevention of back-fires or reversals of the flow of current within the rectifier.

Another object of the invention is the provision of means for suppressing such backfires in their incipient stages and thus re storing normal Working before the main circuit breakers iave time to open.

Other and further objects will be pointed out or indicated hereinafter or will become apparent to one skilled in the art upon an understanding of the invention or its em ployment in practice.

- In the drawings accompanying this specification I illustrate diagrammatically several forms in which the invention may be 5 embodied, but it must be understood that these are presented for purpose of illustration only and are not to be interpreted as having the eiiect of limiting the claims short of the true and most comprehensive scope of the invention in the art.

In the drawings;

Fig. 1 is a diagrammatic view of a glassbulb type of rectifier illustrating the underlying principle of the invention;

Fig. 2 is a diagrammatic view of a singlephase metal-cased rectifier with anode shields for the suppression of back-fires;

Fig. 3 is a diagrammatic view of a single phase metal-cased rectifier similar to that Fig. 11 illustrates a further alternative.

During the operation of mercury vapor rectifiers, trouble is frequently experienced in the shape of current reversals or back-fires within the apparatus, particularly when large currents are being carried. In order to prevent these undesirable occurrences it has been proposed to place suitably shaped metal structures or shields in the path of the arc and to connect these shields to a source of electrical potential, the object being to suspend the flow or current to any anode which might assume a negative potential, such anodes being usually the seat of the current reversals which it is desired to prevent. Th s, by maintaining the shield, grid, or equivalent structure at a higher negative potential than the anode which it surrounds, a check is placed on the flow of current from the shield to the anode owing to the opposition of the potential gradient. A disadvantage of an arrangement such as described is that the preventive measures are confined to those anodes which happen to be at a negative potential, no restriction being placed on a reversed current flow to positive, i.e. working anodes. Current reversals or backfires are not confined to negative anodes, however, an anode with a low positive potential being equally liable to become the seat of a back-fire and to permit current to pass into it from an anode of higher positive potential. Under conditions of normal operation no hindrance must of course be offered to the flow of current in the positive anodes, but in the event of a back-fire it is an advantage to suspend the flow of current in all the anodes, whether positive or negative, and thus positively suppress any incipient reversals of current.

The present invention provides a means of suppressing back-fires in multi-anode rectifiers by interrupting the flow of current in all the anodes in the event of a tendency to back-fire manifesting itself. This is effected by impressing a suitable E. M. F. on grids or shields surrounding the anodes. The E. M. F.s are applied to the various grids through the agency of polarised reverse-current relays placed in either the anode or cathode circuits and set to operate, in the event of a back-tire, as far as possible within one A. 0. period so that the flow of current in the rectifier is suppressed before the main circuit breakers have had time to trip. The grids are charged by being connected to a source of potential of a magnitude suflicient to effectively suppress the flow of current to the anodes. Thus, the flow of current everywhere'within the rectifier may be effectively suppressed on the occurrence of a back-fire by applying to each anode grid a voltage 180 out of phase with that of the anode with which it is associated. The grids of those anodes which were for the time being at a negative potential would thus receive a positive charge and those of the positive anodes a negative charge. In every case the voltage gredient is opposed to the current for a portion of the cycle, thus entirely suspending the flow.

It may be desirable to allow the phase of the grid voltage to vary somewhat from the value stated above. This may be done, the condition to be observed being that the voltage on the grid may not vary in phase by more than 90 from the theoretically desir able value, i. e. 180 out of phase with the associated anode. Therefore, the term opposed in phase, as used in the claims, is intended to mean conditions whereat the voltages on the grids are out of phase with the voltages on the respective anodes between the limits 90 and 270. The fiow of current in all anodes may also be effectively suppressed in the event of a back-fire by impressing E. M. F.s on the anode grids such that the potential of each grid is negative to the anode with which it is associated. The application of a negative E. M. F. to all the grids will also suppress the flow of current. The magnitude of the E. M. F. necessary is usually a matter of experience and depends on local conditions. The necessary degree of adjustability may be obtained by connecting the grids to tappings on the transformer winding (either the main transformer or an auxiliary transformer may be used for this purpose). It will be obvious that the application of a negative charge to the anode grids will not provide a certain means of preventin g heavy current reversals if the grids themselves are capable of carrying comparatively large currents and thus taking the place of anodes. This may be prevented by inserting resistance in the grid circuits, thus limiting the current to a safe value. These resistances fulfill a very important function and they are also necessary when the grids are charged from a separate direct-current source. In such cases the grids are connected via the resistances to the negative pole of the direct-current source, the positive pole of which is connected to the cathode of the rectifier. Another method of rendering the grids incapable of carrying a reversed current due to a baclnfire is to make it impossible for a current to pass directly from the arc to the grid or its equivalent. This may be done by enclosing the grids and the lead conductors connected with them with an insulating sheath. A simple and effective method of securing this result is to enamel the parts in question, although under the large temperature fluctuations inevitable when rectifiers are operated intermittently there is some danger of the enamel cracking and thereby losing in part its protective properties. An alternative method of preventing the passage of current between the a c and the grids is to arrange the latter in such a way that they are entirely removed from the path of the arm. This may be done by making the grids or their equivalents in the form of metal rings or sleeves encircling the insulating shields placed round the anodes for the purpose of guiding the are. These metal rings must fit closely to the insulating shields, since in this case they act capacitively on the space surrounding the anodes.

An understanding of the invention will be most quickly obtained by reference to the accompanying drawings, in which Figs. 1, 2, and 3 illustrate single-phase rectifiers equipped with suitable metal shields or grids and means for charging these. In each case the showing is diagrammatic only.

Referring to Fig. 1, let be understood that the reference numeral 1 denotes the secondary winding of a transformer supplying the glass-bulb type rectifier '7; 2 and 2 denote the anodes, 4 the mercury pool. cathode, and 5 and 5 metal sleeves which surround the arms 11 and 12 containing the anodes and may be put into connection with the transformer secondary winding by means of the switches 8 and 9. The resistance 10 represents the load on the rectifier. On the occurrence of a back-fire in the example considered, all that is necessary to interrupt all the circuits inside the rectifier is to close the switches 8 and 9, thus connecting the sleeve 5 with the anode 2 and the sleeve 5 with the anode 2. By giving these metal sleeves a potential which is in each case opposed in phase to that of the anode associated therewith, the passage of current from anode to cathode or from anode to anode is rendered impossible, owing to the E. M. F. opposed to the are.

Fig. 2 of the drawings represents a metalcased rectifier in which a like result is obtained in a similar manner. The same reference characters are used, with the following differences. There are no anode arms, and the anodes 2 and 2 are located in the rectifier casing and carry shields 21 and 21 of insulating material to the outside of which the metal. sleeves 5 and 5 are secured. Current-limiting resistances 13 and 1d are inserted in the leads connecting the sleeves to the transformer winding 1, these resistances lying between the said sleeves and the switches 8 and 9. The presence of these resistances prevents the reversed current incidental to a back-fire from flowing in the grid circuit when its path to the anodes is interrupted.

Fig. 3 shows a metal-cased rectifier, the arrangement differing from Fig. 2 in the use of metal anode shields andthe presence of an exciting circuit comprising an exciting transformer 15, exciting anodes 16, 17, a resistance 18, and a reactance 19. The resistances are for current-limiting purposes, and the exciting arrangement is intended to ensure the re-ignition of the arc after the clearing of the interruption incident to the suppression of a backfire.

Fig. 4 illustrates the application of the invention to a polyphase rectifier, and the addition of means for automatically applying the grid potentials on the occurrence of a back-fire. In the Figure, the reference character '1 denotes the secondary winding of a six-phase transformer supplying the rectifier 7 which contains six anodes. A reverse-current relay 3 is placed in each anode circuit, and is arranged to operate the switch 6. Resistances 13 are provided to limit the grid currents. 2 denotes the anodes, l the cathode, 27 the anode shields which take the place of grids in this example, 9 the main circuit breaker on the direct-current side, and 10 a liquid resistance representing the load. On the occurrence of a backfire, the relays 3 come into operation and close the switch 6 within a time which is required to be less than the duration of one period of the primary current. The anode shields 27 are thus connected each to a source of potential opposite in phase to that of the respective anode. This, as previously explained, immediately interrupts the reversed current due to the backfire, the relays 3 then returning to their initial position to permit opening of switch 6. The shields are thus disconnected from the source of E. M. F. and the arc is re-ignited, after which the normal operation of the rectifier is resumed. The rectifier load is represented by a resistance 10 in Fig. but in Fig. 5 it is assumed to comprise a direct-current motor 10, the armature of which generates a back E. M. F. opposed to the I). M. F. of the rectifier. In such a case, the purpose of the invention is fulfilled by a single reversecurrent relay 3 connected to a shun 22 in the cathode lead. The same reference characters are used as in Fig. 4.

In the examples illustrated in Figs. 4 and 5, the anode shields are charged by an alternating E. M. F. which is always opposite 1n phase to that of the associated anode. Fig. 6, however, shows an arrangement in which the E. M. F. applied to all the shields is the same, and is obtained from a direct-current source. This is provided by the battery 20, the negative pole of which is connected with one terminal of the switch 6 and the positive pole with the cathode 4. The E. M. F. of the battery must be such that all the internal circuits of the rectifier are interrupted when the switch 6 is closed. Current-limiting re sistances 13 are inserted in the leads to the anode shields for the reasons given above. The main circuit breaker 9 is often fitted with overload relays which operate with a definite time delay. It is possible to arrange that the time of operation of the switch 6 is so short that the main circuit breaker will not be tripped on the occurrence of a back-fire.

Satisfactory operation of the arrange ments described. above for preventing the development or effecting the rapid suppression of back-fires or current reversals in vapor rectifiers is very diflicult to obtain when the rectifier is working in parallel with other direct-current machinery containing inductance, such as direct-current generators or shunt-wound motors (see Figs. 5 and 6). The same holds good when operating with other rectifiers or batteries, if the connecting leads possess considerable inductance. High inductance in the leads may be caused either by their length or by the presence of choke coils. When a back-fire occurs in a rectifier which is operating in parallel with other direct-current machines, a back flow of current will take place from the latter through the rectifier and the phase of the transformer connected to the faulty anode. The rise of this current is shown in Fig. 7 of the drawings, from which it will be seen that the reverse current It does not pass through zero, although the voltage in the phase in which the back-fire occurred is sometimes higher than that of the direct-current machine operating in parallel with the rectifier. The are is not extinguished, therefore, unless the reverse current is interrupted by means of a circuit breaker with overload trip placed in the line connecting the rectifier and the other machine. Such a circuit breaker requires to be reclosed by hand, however, so that its presence does not contribute to continuity of operation.

The present invention ensures uninterrupted operation of the rectifier by the provision of means for momentarily interrupting the reverse current in the connecting line on the occurrence of a back-fire, with the result that the back-fire is cleared and normal operation of the rectifier is immediately resumed. An automatic circuit breaker fitted with a damping device to delay its closing is employed for this purpose. The present of the damping device is necessary to ensure that the breaker does not reclose before the operation of all anodes has been suspended by the act-ion of the charged grids.

Moreover, it has been shown that if a backfire occurs a rd a i stance is inserted in the circuit as the reveise current from the paralleled dire-c urrent machine commences to rise, this reverse cm nt will pass through zero as shown in 2;. 8. This will result in the suppression of the bacl fire, which is only maintained by the flow of the reverse current. A circuit breaker is therefore inserted in the line connecting the rectifier with the direct-current machine with which it is operating in pa allcl, the contacts of this cirshunted by a resistance. cc of the connecting line can thus raised before breaking the reverse currei (l aiteri ds reduced to its former value. The automatic breaker 24 shown in 9 des gned to open on overload and thereby t the resistance 23 in the line connecting L118 rectifier w h the direct-current mach ie 10. Tl c breaker 2 .2 is provided with a by l is of hich it is reclosed immedia y the reverse current has fallen to zero. A switch which operates in response to current variation is most suitable for this p gose. The advantage of such a switch is that o erates within 1/ lOOOth of a second of the sti i of a bac fire in the rectifier, and reeloses immediately the back-fire is cleared.

The resistance 23 mentioned above may rvlzit its function is to be efiectively p: able in cert second brea s, for example, when a ided f r clearing shortcircuits. An arr ;enient or the hind contemplated is illustrated in Fig. 10. the reference numeral 2 ldenotine' the switch for insci-ting the re tance, and 9 the second breaker which is e l in series with 10. It may '31 at the cur nt is so diminished by reason of the large resi n nos 23 that the breaker 9 not operate under any circumstances, e en in the event of a shoitcircult. ice with the invention, however, the, res nce is kept small. and a second r stance 25 connected in shunt with a rect. ier by the operation of the switch Thus, on the operation of the highspeed switch 24, the current from the direct-- current machine flows through the resistance 23andthen divide-s, a part flowing through the rectifier 7, and the remainder through the resistance The currents, in the two parallel branches will be inversely proportional to the resistances or back t. M. F.s of

resistance 23. The resistances 23 and 25 must be adjusted to suit the particular operating conditions in each case. The switch 24 should be preferably constructed to return to its initial position as soon as the reverse current has fallen to zero, thus enabling the operation of the rectifier to be continued without interruption. The desired result may also be obtained by arranging the switch to return to its initial position after the elapse of a definite time, which may be approximately equal to one alternating-current pericd.

In Figs. 9 to 11, the anode grids and chargarrangements are not shown, the object bemg to simplify the drawing. They may be assumed to be arranged in the same manner, however, as shown in Figs. 4 to 6.

The automatic switch with. shunt resistance described above for interrupting the back flow of current and ensuring continuity of operation of a rectifier working in parallel with other direct-current machines or apparatus may be replaced by a valve, such as a or metal-cased vapor apparatus. An unple of such an arrangement is illustrated *ig. 11. The valve 26 is constantly excited and permits current to flow in one direction only, that is, away from the rectifier. Flo current can flow into the rectifier from the direct-current machine 10. By constantly exciting the valve, the rectifier is enabled to resume normal current supply as soon as the back flow of current has been suppressed, and tius continuity of operation is ensured. The valve or the high-speed resistance-shunted switch described above may be placed. either in the positive or in the negative lead of the rectifier.

lVhat I claim is:

1. In a rectifier installation the combination with rectifier having a plurality of 0 .es, of metal grids associated with said n nodes. and means responsive to a current ueously connecting each grid to a source of EQM. F. which differs in phase from that of the anode with which. the grid is associated by more than 3. In a rectifier installation, a rectifier having a plurality of anodes, directcurrent apparatus, connecting leads joining said rectifier and said apparatr. grids associated with the said anodes, means responsive to current reversals within the rectifier for applying E. M. F.s simultaneously to all the grids, the

E M. F.s being opposed in phase to that of the respective anodes, a resistance, switching means responsive to current reversals in the leads joining the rectifier with the direct-current apparatus for inserting said resistance in series in one of said leads, and means connected with said switch for disconnecting the resistance and restoring the leads to their normal condition on the suppression of the current reversal.

4:. In a rectifier installation, a. rectifier having a plurality of anodes, direct-current apparatus, connecting leads joining said rectifier and said apparatus, grids associated w1th the rectifier anodes, means responsive to current reversals within the rectifier for applying E. M. F.s simultaneously to all the grids, the E. M. F.s being opposed in phase to that of the respective anodes, a resistance connected in one of the aforesaid connecting leads, a high-speed switch adapted to short circuit the above resistance, means responsive to current reversals in said connecting lead for opening the high-speed switch, and reclosing means adapted to reclose the said switch when normal current flow is resumed.

5. A rectifier combination as set forth in claim t in which the high-speed switch there referred to is provided with means adapted to delay its operation.

6. A rectifier combination as set forth in claim 4, further characterized by a second resistance and in which the operation of the high-speed switch under the influence of a current reversal simultaneously inserts said second resistance in parallel with the rectifier.

7. In a current-i'ectifyiiig system, metalvapor rectifying apparatus having anodes and a cathode, a direct-current circuit form- .ing part of said. system and including direct-current-consuming apparatus, a circuit breaker disposed in said circuit and having a predetermined tripping-time period, a source of electromotive force, grids associated respectively with said anodes, switch means operable to provide a connection between said source and the respective grids such as to impress upon the latter electrical potential opposed in phase to that of the respective anodes with which they are associated, and reverse-current apparatus having an operating-time period appreciably less than the tripping-time period of said circuit breaker and being responsive to conditions of reverse-current flow in said system and being operable upon occurrence of such conditions to effect closing operation of said switch means.

8.. In a current-rectifying system,-a metalvapor rectifying apparatus having anodes and a cathode, a source of electromotive force, grids associated I respectively with said anodes, switch means operable to provide a connection between said source and the respective grids such as to impress upon the latter electrical potential opposed in phase to that of the respective anodes with which they are associated, and reverse-current apparatus responsive to conditions of reverse-current flow in said system and operable upon occurrence of such conditions to effect closing operation of said switch means.

9. In an electric current rectifying system, in combination with an electric current rectifier subject to reverse flow of current therein, comprising a plurality of anodes, a plurality of control electrodes associated respectively with said anodes, and a cathode, a work circuit connected with said rectifier to be supplied with current therefrom, and a source of alternating current connected with the said anodes for conversion by said rectifier into direct current and the supply thereof to said work circuit, of means actuated responsive to a reverse flow of current in said system operable upon actuation thereof to cause the application of an electro-motive-force simultaneously to all of said electrodes to thereby effect the interruption of said reverse flow of current.

10. In an electric current rectifying system in combination with an electric current rectifier subject to reverse flow of current therein, comprising a plurality of anodes, a plurality of control electrodes associated respectively with the said anodes, and a cathode, a work circuit connected with said rectifier to be supplied with current therefrom, and a source of alternating current connected with the said anodes for conver sion by said rectifier into direct current and the supply thereof to said work circuit, of electro-magnetically actuated means operable responsive to a reverse flow of current in said system to cause the application of an electro-n1otive-force simultaneously to all of said electrodes within a time not exceeding one cycle of said alternating current to thereby effect the interruption of said reverse flow of current.

11. In an electric current rectifying system, in combination with an electric current rectifier subject to the flow of reverse current therein, comprising a plurality of anodes, a plurality of control electrodes associated respectively with said anodes, and a cathode, a work circuit subject to reverse flow of current therein connected with said rectifier to be supplied with current therefrom, and a source of alternating current connected with said anodes for conversion by said rectifier into direct current and the supply thereof to said work circuit, of means actuated responsive to a reverse flow of current in the said work circuit operable upon actuation thereof to cause the application of an electro-motive-force simultaneously to all of said electrodes which is negative relative to the potential of the said cathode.

12. In an electric current rectifying system, in combination with an electric current rectifier subject to reverse flow of current therein, comprising a plurality of anodes, a plurality of control electrodes equal in number to and associated respectively with said anodes, and a cathode, a work circuit connected with said rectifier to be supplied with current therefrom, a transformer having a winding divided into an even number of equally displaced phase sections equal in number to and connected respectively with said anodes operative to supply current thereto from a source of polyphase alternating current for conversion by said rectifier into direct current and the supply thereof to said work circuit, of means included in each of said connections of said winding sections with the respective anodes actuated responsive to and upon the occurrence of a reverse flow of current in any one of said connections to cause the simultaneous connection of each of said electrodes with a section of said Winding opposite in phase to the section of said winding connected with the associated ones of said anodes to thereby impress simultaneously upon each of said electrodes a potential opposite in sign to the potential impressed at the same time on their associated anodes.

13. In an electric current rectifying systern, in combinationwith an electric current rectifier subject to the flow of reverse current therein, comprising a plurality of anodes, a plurality of control electrodes equal in number to and associated respectively with said anodes, and a cathode, a work circuit subject to reverse flow of current therein, the said circuit being connected with said rectifier to be supplied with direct cur-- rent therefrom, a transformer having a Winding divided into an even number of equally displaced phase sections equal in number to and connected respectively with said anodes operative to supply current thereto from a source of alternating current for conversion by said rectifier into direct current and the supply thereof to said work circuit, of electromagnetic means connected in series with said work circuit actuated responsive to and upon the occurrence of a reverse flow of current in the said work circuit to cause the simultaneous connection of each of said electrodes with a phase section of said winding which is opposite in phase to the phase of the section connected with the associated ones of said anodes to thereby impress simultaneously upon each of said electrodes a potential opposite in sign to the potential impressed at the same time on their associated anodes.

14. In an electric current rectifying system, in combination with an electric current rectifier subject to reverse flow of current therein,

comprising a plurality of anodes, a plurality of control electrodes equal in number to and associated respectively with said anodes, and a cathoce, a work circuit subject to reverse flow of current therein, the said work circuit being connected with said rectifier to be supplied with direct current therefrom, a source of alternating current connected with said anodes for conversion by said rectifier into direct current and the supply thereof to said work circuit, of a source of direct current supply having the positive pole thereof connected with the said cathode, and electromagnetic Incans connected in series with said work circuit actuated responsive to and upon the occurrence of a reverse flow of current in the said work circuit operative to cause the simultaneous connection of each of said electrodes with the negative pole of said source of direct current to thereby impress upon each of said electrodes a potential negative relative to the potential of said cathode to thereby cause interruption of the fiow of current between said anodes and said cathode.

15'. In an electric current rectifying system, in combination with an electric current rectifier subject to reverse flow of current therein, comprising a plurality of anodes, a plurality of control electrodes equal in number to and associated respectively with said anodes, and a cathode, a work circuit subjectto reverse flow of current therein connected with said rectifier to be supplied with direct current therefrom, a transformer having a winding divided into an even number of equally displaced phase sections equal in number to and connected. respectively with said anodes operative to supply current thereto from a source of alternating current for conversion by said rectifier into direct current and the supply thereof to said work circuit, of electro-magnetic means included in each of said connections of said winding sections with the respective anodes actuated responsive to and upon the occurrence of a reverse flow of current in any one of said connections to cause the simultaneous connection of each'of said electrodes with a section of said winding opposite in phase to the section of said winding connected with the associated ones of said anodes to thereby impress upon each of said electrodes a potential opposite in sign to the potential impressed at the same time upon their associated anodes to thereby interrupt the said reverse flow of current by way of saidanodes, a current limiting resistance, and electro-magnet means connected in series with said work circuit actuated responsive to and upon the occurrence of reverse flow of current therein operative to effect the connection of said resistance in series with said work circuit to thereby effect the interruption of reverse flow of current in said work circuit by way of said rectifier.

16. In a rectifier installation, the combinetion with a rectifier having a metallic casing, and a plurality of anodes disposed therein, or" metal grids disposed within said casing associated respectively with said anodes, and means responsive to and operable upon the occurrence of reverse flow of current within said rectifier for simultaneously connecting each said grid to a source of E. M. F. displaced in phase with respect to that of the associated ones of said anodes.

17. In a current rectifying system, a metallic vapor rectifying apparatus comprising a metallic casing having a plurality of anodes and a cathode contained therein, a source of electromotive force, grids disposed Within said casing and associated respectively with aid anodes, switch means operable to provide a connection between said source and the respective grids such as to impress upon the latter electrical potential displaced in phase with respect to that of the associated ones of said anodes, and apparatus responsive to reverse current conditions in said system and operable upon occurrence of such conditions to effect the simultaneous connection of said source of electromotive phase with each of said grids.

18. In an electric current rectifying system, in combination with an electric current rectifier subject to reverse fiow of current therein, said rectifier comprising a metallic casing, a plurality of anodes and a cathode contained therein, and a plurality of control electrodes associated respectively with said anodes, a work circuit connected with said rectifier to be supplied with direct current therefrom, a source of polyphase alternating current connected with said anodes for conversion by said rectifier into direct current and the supply thereof to said work circuit,

of means actuated responsive to and upon the occurrence of reverse flow of current in said system operable to cause application of an electromot-ive force simultaneously to all of said electrodes to thereby efiect the interruption of said reverse flow of current.

19. In an electric current rectifying system, in combination with an electric current rectifier subject to reverse flow of current therein, the said rectifier comprising, a metallic casing, a plurality of anodes and a cathode contained in said casing, and a plurality of control electrodes contained in said casing equal in number to and associated respectively wit said anodes, a work circuit subject to reverse flow of current therein connected with said rectifier to be supplied with direct current therefrom, a transformer winding having a plurality of equally displaced phase sections connected respectively with said anodes operable to supply current thereto from a source of polyphase alternating current for conversion by said rectifier into direct current and the supply thereof to said work circuit, of eiectroinagnetically actuated means connected in series with said work circuit operable responsive to and upon the occurrence of reverse flow of current in said Work circuit to cause the simultaneous connection of each of said electrodes with a phase section of said winding which is opposite in phase to the phase section of said w'iiding connected with the associated ones of said anodes to thereby impress simultaneously upon each of said electrodes a potential opposite in sign to the potential impressed at the same time on their associated anodes.

STEFAN VIDMER. 

